This invention relates to a process for production of dihydrocarbyl polysulfides, and more particularly to a process for selective synthesis of dihydrocarbyl trisulfide.
Methods for preparing dihydrocarbyl polysulfides, such as dialkyl polysulfides based on the use of mercaptans and sulfur as raw materials are well known in the art and are described for example in U.S. Pat. Nos. 2,237,625, 3,022,351, 3,275,693, 3,308,166, 3,314,999, 3,340,324, 3,392,201, 3,452,100, 3,755,461, 3,994,979, 4,564,709, 4,876,389, 4,933,481, and 4,937,385; British Pat. Spec. No. 1,160,473; Canadian Pat. Nos 839,767 and 885,990; European Pat. App. Pub. Nos. 25,944 and 337,837; and Japan Kokai (Laid-Open application) Nos. 58-140,063 and 59-10559.
Another approach for producing dihydrocarbyl polysulfides involves oxidizing a mercaptan with air or free oxygen in the presence of a catalyst. In U. S. Pat. No. 2,558,221 the catalyst is a selected natural bauxite which contains on a weight basis 50-70% Al.sub.2 O.sub.3, 8-20% Fe.sub.2 O.sub.3, 2-8% SiO.sub.2, 0.5-5% TiO.sub.2, and 2-30% volatile matter as determined by ignition at 1800.degree. F. In U.S. Pat. No. 2,574,884 the catalyst is alumina associated with a minor amount of vanadia, magnetic iron oxide or chromia. In U.S. Pat. No. 4,277,623 a catalyst system comprising a cobalt molybdate-alkali metal and/or alkaline earth metal hydroxide is used as the oxidation catalyst. And in U.S. Pat. No. 4,288,627 the oxidation catalyst is a supported cobalt molybdate catalyst used in combination with a liquid tertiary amine.
It is also known that dihydrocarbyl polysulfides can be formed by reacting mercaptans with sulfur chlorides such as sulfur monochloride and sulfur dichloride.
Buchholz, et al. describe a continuous process for preparing dialkyl disulfides by reacting an alkyl alcohol and hydrogen sulfide in one reaction zone over a solid particulate catalyst, and then passing the reactor effluent into a second reaction zone where it is reacted as a vapor with elemental sulfur in the presence of the same or a different solid, particulate catalyst.
Of the various dihydrocarbyl polysulfides, dihydrocarbyl trisulfides are particularly desirable for use as antiwear and extreme pressure lubricant additives because of their superior performance capabilities and their generally lower corrosiveness towards "yellow metals" such as copper. Dihydrocarbyl higher-polysulfides (e.g. polysulfides with more than about 3 sulfur atoms per molecule) are less desirable than polysulfides containing 3 or less sulfur atoms per molecule. Hence, one object of this invention is to provide a process which yields additives containing a high percentage of dihydrocarbyl di- and tri-sulfides and less dihydrocarbyl higher-polysulfides.
Another object of this invention is to provide a process for producing dihydrocarbyl polysulfides of the formula, R--S.sub.x --R, where R is hydrocarbyl and x is an integer representing the average number of sulfur atoms in the product and is above 2.0, e.g., at least 2.5, and preferably is in the range of about 2.7 to 3.5.